A laser beam has an intensity spatial distribution with respect to a plane perpendicular to the traveling direction thereof. When a laser beam is used as a light source, it is required to use a center area of the laser beam having a more uniform intensity spatial distribution or to make the un-uniform intensity spatial distribution uniform in any suitable manners.
A laser beam radiating with a multimode which has a very complicated intensity spatial distribution, can illuminate a relatively large object uniformly. However, when a relatively small object to be observed using a microscope is illuminated, a laser beam radiating with a multimode cannot be used as a proper light source as it is, because un-uniformity of the un-uniform intensity spatial distribution becomes visible.
In a laser beam, which radiates with a basic TEM00 mode, the intensity spatial distribution of the laser beam is a Gaussian distribution. Therefore, even if the center area of the laser beam having a relatively uniform intensity spatial distribution is selectively used, the intensity spatial distribution of the selected area cannot be completely uniform and a large portion of the power of the laser beam is consumed wastefully.
When a un-uniform intensity spatial distribution of a laser beam exists, out-of-focus illumination by the laser beam may be used for uniform illumination. However, variations in intensity due to interference of the laser beam cannot be overcome absolutely.
Conventionally, there is known an illumination method using a laser beam in such a manner that a laser beam is radiated on a dispersion element, such as a diffraction grating (refer to non-patent document 1), a mesh screen (refer to non-patent document 2), or an optical diffusion plate (refer to non-patent document 3) to obtain a pseudo light source from the surface of the dispersion element for use in illumination. However, since a laser beam has a high coherence, interference noises called speckle noises occur.
It is to be understood that a phase of the laser beam is not completely disturbed at random by diffusion. A light intensity becomes extremely high at an area where the phase is accidentally in order, while the light intensity becomes zero at an area where the phase is out of order by 180°, which results in generating a spot noise of a granular pattern. This is referred to as a speckle noise.
In order to reduce the speckle noises, a laser beam is circularly rotated with respect to a dispersion element (refer to non-patent documents 1, 2 and 3) or a diffraction grating, screen mesh, or light diffusion plate is moved using a piezo element (refer to non-patent document 4) so that an object is more uniformly illuminated by changing an interference pattern. However, in this method, uniformity is not satisfactory and a large amount of light is scattered.
As an alternative method, there is proposed a method of reducing speckle noises using plural optical fibers having different lengths (refer to patent document 1). However, the bundle structure using plural optical fibers has particularly a large incident loss and the total light transmittance is at most 50%. When the structure is used at resolution power level of an optical system, a huge number of optical fibers are required, and noises cannot be removed substantially.
In contrast, there is proposed a method of combination of a plate for disturbing phase randomly and bundled optical fibers (refer to patent document 2). In this method, an effect for more randomly disturbing phase can be expected, however, total system becomes more complicated and leads to a larger loss of light amount.
There is proposed to divide a single optical fiber into plural branches on the way to realize the similar effect (patent document 3). However, dividing the optical fiber has a large reflection loss of light at the branch position although uniformity can be better.
Patent document 4 discloses an illumination system, which is provided with a sensor for detecting divergent angle of a laser beam to adjust the position of an element for changing coherence of the laser beam of many optical elements in a laser source in accordance with output of the sensor. However, a very complicated optical system and a reference light are required in this system so that the whole loss of light is very large.
A method for deforming a mirror by an actuator to improve uniformity of an intensity spatial distribution of an illumination light is proposed (refer to patent document 5). However, the apparatus is bulky and very expensive and requires delicate and complicated adjustment of optical axis.
Moreover, a method for uniformly illuminating an object using the light leaked from an optical fiber is proposed (refer to patent document 6). However, in this illumination, a high power laser is required to obtain sufficiently bright illumination, because the leaked light is weak.
A method using a prism is proposed (refer to patent document 7). In this method, an accurate oblique incidence to a prism with a high precision optical axis adjustment mechanism is required, and there is a significant reflection less by the oblique incidence. Moreover, in order to reduce the optical loss in the prism, an expensive large prism made of a high purity material equivalent to that for the optical fiber is required.
Moreover, there is known a method for reducing disturbances due to noises by capturing a large number of images at a time or in a time series, while changing the interference pattern of a light, and then averaging the images through image processing. However, in this method, much time is required for image processing and is not suitable for the purpose of real time observation.
There is another method of illumination by scanning a focused laser beam on whole area. In this method, a very complicated and expensive optical system is required for scanning a focused laser beam accurately. The complicated optical system results in a large loss of light amount. Moreover, it is difficult to realize real time observation, because it takes much time for scanning whole area.                [Non-patent document 1]        H. J. Gerritsen, W. J. Hanman and E. G. Ramberg: Appl. Opt., Vol. 7, pp. 2301-2311, 1968        [Non-patent document 2]        G. B. Brandt: Appl. Opt., Vol. 12, pp. 368-371, 1971        [Non-patent document 3]        S. Lowenthal and D. Joyeux: J. Opt. Soc.Am, Vol. 61, pp. 847-851, 1971        [Non-patent document 4]        C. S. Ih and L. A. Baxter: Appl. Opt., Vol. 17, pp. 1447-1454, 1978        [Patent document 1]        Japanese Patent laid-open publication No. Tokkaihei 06-167640        [Patent document 2]        Japanese Patent laid-open publication No. Tokkaihei 11-101925        [Patent document 3]        Japanese Patent laid-open publication No. Tokkaihei 05-217854        [Patent document 4]        Japanese Patent laid-open publication No. Tokkaihei 05-275317        [Patent document 5]        Japanese Patent laid-open publication No. Tokkaihei 09-006011        [Patent document 6]        Japanese Patent laid-open publication No. Tokkai 2000-221332        [Patent document 7]        Japanese Patent laid-open publication No. Tokkaihei 09-159964        